<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        var str = '2136578099999'
        var temp = ''
        
        // 对字符串进行循环遍历，每次取五个字符
        for(var i = 0; i < str.length-5; i++){
            // 去出两个五位数进行比较
            var str1 = str.substr(i,5);
            var str2 = str.substr(i+1,5);
            // 判断出一个最大值
            if(str1<str2){
                temp = str2;
            }
        }

        console.log(temp);
    </script>
</body>
</html>